When you find a picture online that would be perfect for your project, it's hard to know whether you're allowed to use it. Follow this flow chart to know for sure and avoid getting yourself in trouble.
The chart comes from Curtis Newbold, The Visual Communication Guy, and explains copyright, fair use, creative commons and public domain in an easy-to-consult format. It's important to double check before using someone else's image. Just follow the chart and answer the questions and you'll know whether it's cool to use that cat pic for your blog or not. If you're not completely sure, ask for permission or don't use the image. Click the chart for a full-size version.